1. Field of the Invention
The present invention relates to a lens driving apparatus and a lens driving method in a lens-integrated imaging device, a lens-interchangeable imaging device and the like.
2. Description of Related Art
There is a demand for a faster and higher-precision autofocusing along with the increase in velocity and number of pixels of image sensors in imaging devices such as digital cameras. In addition, there is arising a demand for characteristic lenses such as large-diameter and bright lenses, telephoto lenses with long focal lengths, and macro lenses with high-photographing magnifications. Therefore, a driving apparatus of a focus lens is required to move a large and heavy lens, or move the lens quickly over a long stroke, for example.
A stepping motor is often used to drive a focus lens in terms of cost and size. There may occur a phenomenon called step-out, in which the velocity of the stepping motor is too fast relative to the load. Occurrence of a step-out may result in a gap between an instructed position and an actual position, and therefore the maximum velocity is determined so as to secure a sufficient safety factor relative to loads assumed in design.
Attempting to secure a sufficient safety factor in consideration of step-out may fail to exert the highest performance of the stepping motor and therefore is inefficient. Accordingly, Japanese Patent Laid-Open No. 2001-178193 (hereafter referred to as “Patent Literature 1”) has proposed a method of attaching a sensor to a stepping motor, constantly detecting the rotation position of the stepping motor, performing feedback control so that the difference from a motor position command signal becomes zero, and exerting the maximum performance of the stepping motor.
The lens driving apparatus disclosed in Patent Literature 1 performs feedback control, i.e., constant feedback control, of the stepping motor from activation thereof. Performing constant feedback control may lead to an abrupt control of velocity or rotational direction of the motor in an attempt to cancel a temporary increase of the difference between the current position and the motor position command signal caused by a delay of the follow-up of load relative to the rising of a position command signal of the motor due to static friction or inertia. Accordingly, there may occur an excessive decrease of velocity or a temporary reverse phenomenon that may result in unstable feedback control, thereby preventing smooth and quick start of operation. Furthermore, there have been drawbacks such as: taking a long time before reaching a target position located also at a relatively short distance; taking a long time before reaching a target velocity; taking a long time for autofocusing; or generating a noise due to unstable driving.